This invention relates to an improved method for the polymerization of ethylenically unsaturated monomers; and more particularly, to a method for the polymerization of one or more ethylenically unsaturated monomers, other than vinyl chloride, in a heterogeneous phase which is free from the problems of polymer scale deposition.
Heterogeneous polymerization, such as suspension or emulsion polymerization, in an aqueous medium is widely employed for the preparation of a variety of synthetic resins, including polystyrene, polyacrylonitrile, acrylonitrile-butadiene-styrene copolymer, styrene-butadiene copolymer and other synthetic rubbers. The heterogeneous polymerization methods in an aqueous medium are of industrial importance, but usually suffer from polymer scale deposition on the various surfaces in contact with the monomer within the polymerization reactor.
In practice, the inner walls of the polymerization reactor and the surfaces of the agitator and other equipment installed within the polymerization reactor are contacted by the monomer or monomers during a polymerization run and become covered with polymer scale. This results in a decrease in the yield of polymer product as well as in the cooling capacity - and hence the production efficiency - of the reactor. Moreover, the polymer scale may eventually come off the reactor surfaces and enter the polymer product, thereby causing degradation of its quality. Furthermore, the removal of polymer scale after each polymerization run not only requires extensive time and labor, but also causes health problems to workers in view of the toxicity of unreacted monomer or monomers absorbed in the polymer scale. This health hazard has been one of the most serious environmental issues in recent times.
A number of methods have been proposed for the prevention of polymer scale deposition, specifically in the suspension polymerization of vinyl chloride or a monomer mixture composed mainly of vinyl chloride. For example, the alkaline substance is added to an aqueous polymerization medium so that the aqueous medium will have a pH higher than 8.0. In another example, an inorganic oxidizing agent is added to the aqueous medium. In still another example, the inner walls of a polymerization reactor are coated with an organic polar compound or an organic dye before the polymerization run is started. These prior art methods have, in fact, been successful in some cases for the commercial production of polyvinyl chloride resins, but have certain disadvantages.
The method wherein an alkaline substance or inorganic oxidizing agent is added to the reaction mixture not suitable for the polymerization of certain ethylenically unsaturated monomers; such as vinyl esters, esters of acrylic and methacrylic acids and diene compounds, since such addition tends to decompose or oxidize those monomers. Further, the method wherein the polymerization reactor walls are coated exhibits a decrease in polymer scale prevention and a decrease in the duration of this effect, when the polymerization is carried out with an emulsifier added to the aqueous polymerization medium; when the copolymerization of a conjugated diene monomer and a vinyl monomer is intended; or when the polymerization is carried out in the presence of an acyl peroxide (such as benzoyl peroxide or lauroyl peroxide); a higher carboxylic acid (such as stearic acid, an ester or a salt thereof); or a molecular weight controlling agent, such as mercaptans. Thus, it is a generally accepted conclusion that the prior art methods can be satisfactorily used only in the suspension polymerization of vinyl chloride in an aqueous medium of the most simplified. Such a formulation contains a nonionic, polymeric dispersing agent which is free of unsaturations; e.g. partially saponified polyvinyl alcohol or methyl cellulose.